Y-junction circulator with common arm capacitor

ABSTRACT

A Y-junction strip line circulator of the lumped constant type in which a single, common capacitor tunes symmetrically with the inductance of all three circulator arms. The capacitor may be fixed or may be electrically or mechanically variable to tune, adjust or switch the circulator.

United States atent [72] Inventors ReinhardlLKnerr [56] References Cited UNITED STATES PATENTS Allentown;

Daniel .1. 'l'hlbanlt, Whitehall, both of, Pa.

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R o. "m CD m6 cm mm A5 [54] Y-JUNCTION CIRCULATOR WITH COMMON ABSTRACT: A Y-jnnction strip line circulator of the lumped constant type in which a single, common capacitor tunes symmetrically with the inductance of all three circulator arms.

The capacitor may be fixed or may be electrically or mechaniadjust or switch the circulator.

FERRITE MEANS TO VARY CAPACITANCE PATENTED SEP14I9H 3,605,040

I4 GYROMAGNETIC MATERIAL MEANS TO VARY CAPACITANCE FIG. 3

k. h. KNERR WVENTORS 0. J. TH/BAULT A T TORNE V Y-JUNCTION CIRCULATOR WITH COMMON ARM CAPACITOR BACKGROUND OF THE INVENTION This invention relates to Y-junction circulators of the lumped constant variety and, more particularly, to circulators of this type.

The prior art form of lumped constant circulator has been treated analytically in articles by Deutsch and Wieser, IEEE Transactions on Magnetics, Vol. 2, No. 3, Sept. 1966, pp. 278-282; Konishi, IEEE Transactions on Microwave Theory and Techniques, Vol. 13, Nov. 1965, pp. 852-864; Davis and Cohen, IEEE Transactions on Microwave Theory and Techniques, Vol. 1 1, Nov. 1963, pp. 506-512; and disclosed in the patent to Konishi: U.S. Pat. No. 3,335,374 and in the copending applications of the applicant Knerr hereof. Ser. No. 864,371, filed Oct. 7 1969 and Ser. No. 795,907, filed Feb. 3, 1969. In general, the circulator includes a ferrite disk core and three branches or arms each comprising a tuned circuit containing a conductor and a discrete capacitive element. The conductors are radially disposed on the top surface of the core, with one end grounded in a manner providing threefold symmetry. When connected to sources of high frequency energy, these conductors exhibit inductive reactances; therefore, the discrete capacitors are added to each arm to achieve the resonance necessary for flux inducement in the core. While it would appear that it is possible to vary the frequency of operation of the circulator by adjustment of these capacitors, such a procedure is effective only over a very narrow band and even within this band slight differences in adjustment of different arms can create a critically undesirable imbalance in the circulator.

SUMMARY OF THE INVENTION In accordance with the present invention it has been found that if the three inductive arms of the circulator are initially symmetrical, all may be tuned to resonance by a single discrete capacitor common to and therefore symmetrical to all three arms. The resulting single tuned circuit can have a wider bandwidth than the form involving three separate resonant circuits. If the single capacitor is variable, the frequency of operation of the circulator may be tuned from range to range by a single adjustment without in any way affecting the symmetry between the arms.

The present invention is particularly applicable, though not limited, to the circulator of the type described in more detail in the above-mentioned copending application, Ser. No. 864,37l,filed Oct. 7,1969,in which each branch conductor is split into at least two split conductors which intermesh with each other to produce a plurality of crossings arranged so that the pattern of overcrossings and undercrossings maintains symmetry for the wave currents and voltages. In accordance with a particular embodiment, each of these strip conductors is terminated in an intermediate potential plane located above the usual ground plane. The common capacitor, either fixed or variable, is connected between the intermediate plane and the ground plane.

In accordance with a further feature of the invention, all or part of the common capacity may be supplied by an electrically controllable capacitor, such as a varactor, to provide an electrically tunable circulator. Finally, if by switching the capacitor by such an increment that circulator operation shifts from one mode of operation to another, the direction of circulation can be reversed. This provides a switching circulator capable of much faster operation than those which depend upon reversal of the biasing field.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top view of one embodiment of the present invention;

FIG. 2 is a sectional perspective view taken along the line 2-2 in FIG. 1;

FIG. 3 is a schematic diagram showing the equivalent variable common capacitor;

FIG. 4 is an eigen value diagram required to explain the operation of the invention;

FIG. 5 is a schematic diagram showing how the common capacitor may be made electrically variable or switched to switch the direction of circulation; and

FIG. 6 is an eigen value diagram required to explain the operation of the invention.

DETAILED DESCRIPTION Referring to FIGS. 1 and 2, the circulator includes three conductors ll, 12 and 13, arranged in rotational symmetry on the upper face of substrate 14 composed of a gyromagnetic material, commonly a ferrite or a composite alumina-ferrite. The middle segment of each conductor is divided into at least two split conductors, such as 19 and 20 of strip 12, or 21 and 22 of strip 11, or 23 and 24 of strip 13, and the insulated crossing points of these split conductors are arranged as shown in FIG. 1 to induce a symmetrical magnetic field inside the gyromagnetic material. More particularly, the desired pattern is preferably formed upon the ferrite substrate 14 by thin film, photolithographic processes well known in the art of printed circuits. Thus, a first pattern is applied which includes the main portion and two parallel strips of the split portion for each of the three conductors 11, 12 and 13. Each strip portion, such as 20, has gaps, such as 25, at the points at which that strip is to form an overcrossing with strip 21 and is continuous, as at 26, where it is to form an undercrossing with strip 23. Each undercrossing strip is then covered at the crossing by a thin layer of nonconducting or dielectric material, such as layer 27, over strip 21. Conductive material, such as 28, is then applied over the dielectric 27, making a conductive bridge having conductive contact with each of the ends of strip 20 on either side of gap 25 and completing the insulated overcrossing of strip 20 over strip 21.

Intermediate potential plane 16 is located next to the bottom face of ferrite substrate 14 and may be, for example, a conductive film applied as above to substrate 14. Conductive members 15, in the form of probes, rivets or screws, connect one end of each conductors ll, 12 and 13 to plane 16 so that the conductors are arranged with their free ends apart. A ground plane 17 is suitably spaced from intermediate plane 16, as by dielectric spacer l8. Plane 17 is continuous with or at least connects to the ground planes associated with the circuits connected to free ends of conductors 11, 12 and 13, so that when any one of these is excited as an input by high frequency electromagnetic wave energy relative to ground plane 17, the two strips into which that conductor is split will be simultaneously excited while the remaining free end of the next successive one of strips 11, 12 or 13 becomes the output port.

In the form illustrated in FIG. 2 a capacitor common to branches 11, 12 and 13 is formed between intermediate plane 16 and ground plane 17 by a conductive plate 31 electrically connected to plate 17 by a screw 32 so that the spacing between plane 16 and plate 31 may be varied to vary the capacity therebetween. In order to enhance the effect that the capacity formed by plate 31 has upon the circulator, it is possi ble to minimize other capacities in the circuit such as those formed at the strip crossings. This is easily done by reducing the opposing areas of the faces of the strips at a crossing and/or increasing their spacing. For certain applications as will be described hereinafter, capacity from each port to ground either with or without capacity between the ports can be used in combination with the common arm capacity.

The resulting circulator is shown schematically in FIG. 3. Thus, ends of each of the three circulator arms are interconnected by means of connection 36 corresponding to intermediate plane 16. Capacity 37 is connected from the point of interconnection to ground The circuit of FIG. 3 serves to indicate that while the form of intermediate plane 16 shown in FIG. 2 is one having physical convenience, the required interconnection may be made by any symmetrical circuit of other design which connects the intermediate ends and that the capacity 37 may be a lumped trimmer capacitor of conventional design or merely a lumped fixed capacitor.

Furthermore, while the structure of FIG. 2 is in the form of unsymmetrical transmission line, it should be recognized that a second ground plane and intermediate plane either with or without a second body of ferrite can be located above the structure as shown to produce a more symmetrical transmission line structure. Such a plane would still be schematically represented by connection 36 of FIG. 3. It is then possible to locate other reactive elements between the second intermediate plane and the second ground plane which may or may not duplicate capacitor 37 but would in effect be connected in parallel with it.

The circuit of FIG. 3 may now be analyzed by defining its eigen vector excitation in the manner cited in the above-cited publications. The current i at each port and voltage V at each port produced thereby with C shorted are expressed by Deutsch et al. as

From the impedance matrix (Z) the eigen values A can be found from use of the eigen vector a as shown by Montgomery et al. in Principles of Microwave Circuits, Dover Publishing, Inc. pp. 405-407,resulting in:

r (l l u[ o l n[ ])l ..]=0 where [I] is the unity matrix and l zx (10) Introducing the effect of C in accordance with the present invention, the eigen values using equations (2) and (7) become Substituting the values of Equations 3, 4 and 5 in Equations 11,12 and 13, the following are obtained:

)\ I: 1 00C (17a) refresenting the three eigen values for the circuit of F G. 3

The reflection coefficients of these eigen values are It is well known that the eigen vectors correspond to the three modes necessary to make a nonreciprocal three port act as a circulator, that circulation occurs at those frequencies for which the reflection coefficients of equations (l7), (l8), and (19) are apart in phase, and that the direction of circulation is determined by the sequence of the eigen values. For verification of these principles, reference may be had to the foregoing publications.

Applying these principles to the present invention it should be noted that all three eigen values and their reflection coefficients are frequency dependent, but that only A, depends on C. Thus, the phase of A may be adjusted independently of the phase of A, and A,. It is directly from the independence of this adjustment that the advantages of the present invention derive.

In FIG. 4 the solid vectors 41, 42, and 43 represent reflection coefficients of the eigen values A,, A and A, respectively, in a condition producing circulation at a first frequency f,. The required phase of A, is the result of a first value of C, of capacitor 37. Circulation in the direction A,A,-A is shown by the arrow 47. As the applied frequency changes all three vectors rotate. In prior art structures A, and A, rotate at a different rate than A, so that a frequency change rapidly produces departure from the required 120 phase difference and circulation ceases. The dotted vectors 45 and 46 represent possible new positions for A, and )t i In accordance with the present invention, the effect of common capacitor C can make}, track A and A to agreaterextent than the circuit involving separate resonant circuitsfor each branch. Thismeans that a frequency shift substantially greater than the above-mentioned given shift is required before circulation ceases. When circulation does cease, however, A, can be returned to a new value by changing capacitor 37 to a value C as represented by the dotted vector 44 such that an approximate 120 phase shift again exists relative to vectors 45 and 46 of A, and A respectively. Such returning restores circulator action. Thus, a fixed common capacitor in accordance with the present invention can have a wider bandwidth than the prior art form involving separate resonant circuits, and when variable the common capacitor can extend the range of circulation by tuning.

The foregoing analysis has assumed that the common arm capacity C is the only significant capacity. There may be, in addition, capacity from each port to the ground plane, capacity from each port to the intermediate plane, and capacity between ports. The crossover capacity mentioned hereinbefore is a combination of all of these. The effect of capacity to the intermediate plane affects only A and k This then does not affect the separate degree of control over A,. Capacity from each port to ground affects A, and M as well as A, (this being the predominant capacity in the usual prior art structure) and together they control the operating frequency. Thus, if it is intended to tune by control of A the presence of capacity to ground dilutes the desired effect. If control of is desired for other reasons to be set out hereinafter, control of frequency may then be made to depend upon A and M.

In FIG. 5 the common capacitor comprises the series combination of capacitor 51 and a device 52, such as a varactor, having a capacity dependent upon its bias. A suitable variable bias is schematically represented by source 54. Capacitor 51 is preferably large compared to the capacity produced by varactor 52 and is included primarily to block the DC biasing current from the circulator. Inductance 53 connected in series with source 54 isolates high frequency currents from source 54 and maintains a high impedance at the high frequencies across varactor 52. If isolation is not required or if isolation is provided at other points in the system, capacitor 51 and/or inductance 53 may be eliminated. Variation in the current from source 54 can thus change the center frequency of the circulator.

Referring now to FIG. 6 it should be recalled that there are in fact a first frequency f for which A, and A are 120 apart as considered in connection with FIG. 4, and now represented in FIG. 6, by the solid vectors 61 and 62 as well as a second frequency f for which these vectors are 240 apart as represented by the dotted vectors 63 and 64. The prior art has recognized that circulation is possible at either frequency. Circulation is produced in the order k h-A as represented by the arrow 65 when A, bears the required 120 relationship to A, and A as represented by the solid vector 66 and in the reverse direction as represented by the arrow 67 when A, is as represented by the dotted vector 68. If initially the circulator is properly designed or if provision is made for returning it along with the switching so that the frequency spacing between f and f is no greater than twice the bandwidth of the circulator, there will be a frequency band in between for which direction of circulation will be reversed as a result of the shift. Such retuning can, for-example, be obtained by varying the capacity from each port to ground as set forth above. This capability is shown in FIG. 5 by the switch 55 which is effect shorts out varactor 52 so that the net common capacity can be abruptly'changed from the value locating A, from that at 66 to that at 68, thereby reversing the direction of circulation from that shown by arrow 65 to that shown by 67. Obviously, a square wave of voltage applied to varactor 52 would have the same effect on the total capacity as does switch 55.

It should be understood that the arrangement of capacitors, varactors, etc. of FIG. 5 is merely illustrative of the principles of the invention and is not intended to represent the only practrcal example. Clearly there are many ways In which the value of the common capacitor can be varied either gradually in response to a control signal or in discrete changes. Further, it should be recalled that changing the value of one capacitor in parallel with another has a greater effect on the total than when the components are arranged in a series circuit as shown.

What is claimed is:

1. A circulator for operation in a given band of high frequency electromagnetic wave energy comprising a core of magnetically biased gyromagnetic material, at least one ground plane, a plurality of conductive strips intermeshing at to each other across said core and forming parts of three strip transmission lines which exhibit inductive reactance when one end is excited by high frequency wave energy relative to said ground plane, the other ends of said strips being interconnected at an intermediate point separated in space and potential from said excited ends and from said ground plane and from the intermeshing portions of said strips, and means for introducing capacitance between said point of interconnection and said ground plane of such value to interact with the inductive reactance of the strips to produce circulation within said given band.

2. The circulator according to claim 1 wherein said means for introducing capacitance is variable.

3. The circulator according to claim 2 wherein said means for introducing capacitance includes a varactor.

4. A circular comprising a core of magnetically biased gyromagnetic material, a ground plane, a plurality of conductive members adapted to form parts of three transmission lines by having one end of each adapted to be excited by high frequency wave energy relative to said ground plane, said conductive members crossing over each other and having the other end of each interconnected at a point having a potential separated from said ground plane, said conductive members being further inductively coupled to each other by said core in the region of their crossing between said ends, and means for introducing a lumped capacitance between said point of interconnection and said ground plane.

5. The circulator according to claim 4 wherein said means for introducing capacitance includes means for abruptly changing the value of said capacitance between two discrete values, one of said values being selected to produce circulation between said lines in one sequence and the other to produce circulation between said lines in a reverse sequence. 

1. A circulator for operation in a given band of high frequency electromagnetic wave energy comprising a core of magnetically biased gyromagnetic material, at least one ground plane, a plurality of conductive strips intermeshing at 120* to each other across said core and forming parts of three strip transmission lines which exhibit inductive reactance when one end is excited by high frequency wave energy relative to said ground plane, the other ends of said strips being interconnected at an intermediate point separated in space and potential from said excited ends and from said ground plane and from the intermeshing portions of said strips, and means for introducing capacitance between said point of interconnection and said ground plane of such value to interact with the inductive reactance of the strips to produce circulation within said given band.
 2. The circulator according to claim 1 wherein said means for introducing capacitance is variable.
 3. The circulator according to claim 2 wherein said means for introducing capacitance includes a varactor.
 4. A circular comprising a core of magnetically biased gyromagnetic material, a ground plane, a plurality of conductive members adapted to form parts of three transmission lines by having one end of each adapted to be excited by high frequency wave energy relative to said ground plane, said conductive members crossing over each other and having the other end of each interconnected at a point having a potential separated from said ground plane, said conductive members being further inductively coupled to each other by said core in the region of their crossing between said ends, and means for introducing a lumped capacitance between said point of interconnection and said ground plane.
 5. The circulator according to claim 4 wherein said means for introducing capacitance includes means for abruptly changing the value of said capacitance between two discrete values, one of said values being selected to produce circulation between said lines in one sequence and the other to produce circulation between said lines in a reverse sequence. 